1. Field of the Invention
The invention relates to the field of power electronics. It relates to a method of manufacturing a semiconductor element having a cathode and an anode from a wafer.
2. Description of the Related Art
To achieve the best possible electrical characteristics for semiconductor power switches, such as an IGBT (Insulated Gate Bipolar Transistor), the thickness of the active zone of a semiconductor element must be selected to be as close as possible to the physical material boundaries.
By way of example, the thickness has a direct effect on on-state losses. In the case of breakdown voltages of 600-1800 V, semiconductor element thicknesses of 60-250 xcexcm are therefore desirable. However, such small thicknesses are a big problem in the production of semiconductor elements, because wafers having a diameter of 100 mm and more should have a thickness of at least 300 xcexcm in order to minimize the risk of breakage during manufacture.
Up to now, this problem has been solved by the so-called epitaxial technique. This involves growing an electrically active region on a mounting substrate having a relatively large thickness of 400-600 xcexcm. The mounting substrate then firstly ensures the necessary robustness for the semiconductor element produced, and secondly the substrate forms the anode of the semiconductor element.
Generally, there is a barrier layer, also called the buffer, arranged between the mounting substrate and the electrically active region. In the off-state case, the barrier layer serves to decelerate the electric field abruptly before the anode and thus to keep it away from the latter, because, if the electric field were to reach the anode, the semiconductor element would be destroyed. Growing the active region is a lengthy and complicated process, so that this epitaxial technique is relatively expensive. Furthermore, this technique has the disadvantage that it is not possible to dope the mounting substrate, that is to say the anode, sufficiently weakly. This would be an advantage, however, because the anode of a power semiconductor element should be doped as weakly as possible in order for it to obtain ideal electrical properties. Weak doping means high resistivity, however, which, with the relatively large thickness of the mounting substrate, would result in a not negligible resistance value.
A relatively new method of manufacturing a semiconductor element has therefore come to light which requires no epitaxial layers. Such methods are known, for example, from Darryl Burns et al., NPT-IGBT-Optimizing for manufacturability, IEEE, pages 109-112, 0-7803-3106-0/1996; Andreas Karl, IGBT Modules Reach New Levels of Efficiency, PCIM Europe, Issue 1/1998, pages 8-12 and J. Yamashita et al., A novel effective switching loss estimation of non-punchthrough and punchthrough IGBTs, IEEE, pages 331-334, 0-7803-3993-2/1997. Semiconductor elements manufactured using this method are called NPT (non-punchthrough), in contrast to the punchthrough semiconductor elements based on the epitaxial method.
In this method, a relatively thick wafer without any epitaxial layer is used as the starting material. Typical thicknesses are 400-600 xcexcm. In a first step, the wafer is treated on the cathode side, that is to say photolithography, ion implantation, diffusion, etching and other processes necessary for the manufacture of the semiconductor element are carried out. In a second step, the wafer is reduced to its desired thickness on the side opposite to the cathode. This is done using customary techniques, generally by grinding and etching. In a third step, an anode is then diffused in on this reduced side.
Although this method is distinguished from the epitaxial method by its lower costs, it nevertheless also has a plurality of disadvantages:
Diffusion of the anode is relatively difficult because, in this method step, the wafer is already very thin and can thus break easily. In addition, the element must no longer be heated intensely, because, in the first method step, metal layers which melt at temperatures above 500xc2x0 C. have already been applied on the cathode side. This means that the anode can be doped only weakly. This could, admittedly, have a positive effect on the electrical properties of the semiconductor element. However, as it is not possible to incorporate a sufficiently high doping quantity, which could be used as a buffer, the semiconductor element must be thick enough to ensure that avalanche breakdown occurs in off-state mode before the electric field reaches the anode. In principle, semiconductor elements manufactured in this manner are therefore thicker than elements manufactured using the epitaxial technique. This means that the advantage of the weakly doped anode is at least partially cancelled out by the aforementioned disadvantages of too thick an active region.
EP-A-0,700,095 additionally discloses a turn-off thyristor suitable for high off-state voltages. This thyristor comprises a semiconductor element having an anode and a cathode, the anode having a transparent emitter. Such anode emitters are already known for low-power components such as solar cells, diodes or transistors. A transparent anode emitter is understood as being an anode-side emitter with comparatively weak injection, so that high proportions of the electron current coming from the cathode can be extracted without recombination and thus without releasing an injected hole. In front of this transparent anode emitter, there is a barrier layer which, firstly, reduces the electric field in off-state mode, but, secondly, can also be used to influence the injection efficiency of the transparent anode. In this case, the barrier layer is either diffused in or is produced epitaxially, the doping profile in the first case having a Gaussian distribution and, in the second case, having a distribution which is homogeneous or step-like over the layer thickness. Although this semiconductor element exhibits positive behavior in the operating state, it can similarly not be manufactured with any desired thickness on account of the risk of breakage.
It is therefore the object of the invention to create a semiconductor element which is as thin as possible and can be manufactured economically.
This object is achieved is achieved by a method of manufacturing including the steps of a) doping the wafer from the side opposite to the cathode producing a doping profile, b) treating the wafer on the cathode side, c) reducing the thickness of the wafer on the side opposite to the cathode to such an extent that the doping profile is removed except for a weakly doped end region of the doping profile, the weakly doped end region forming a barrier region, and d) producing an anode on the side of the wafer on which the thickness was reduced.
The method according to the invention combines the advantages of a semiconductor element manufactured using the epitaxial technique and of a semiconductor element manufactured using the NPT technique, the result being a semiconductor element whose electrical properties are clearly distinguished from those of the semiconductor elements manufactured using these two known methods.
According to the invention, the procedure is as in the NPT technique without epitaxial layers, a barrier region being added before the starting material is treated on the cathode side. The barrier region is added by doping from a side of the wafer which is opposite to the future cathode, and this produces a doping profile whose density increases toward the future anode and which has a cut off doping profile. After the processing on the cathode side, the wafer is thinned to such an extent that the doping profile is removed down to a weakly doped end region which essentially forms the barrier region. A weakly doped anode, preferably having a transparent anode emitter, can then be manufactured which is protected, in off-state mode, against the electric field by the adjacent, preferably adjoining, barrier region.
A further advantage is that the semiconductor element 6 according to the invention has a positive temperature coefficient for the voltage drop in on-state mode, unlike the elements in the epitaxial technique.
The method according to the invention can be used to manufacture the widest variety of semiconductor elements, particularly for IGBTs (Insulated Gate Bipolar Transistor), GTOs (Gate turn-off Thyristor) or conventional thyristors.
Other advantageous embodiments can be found in the dependent patent claims.